The collection consists of Omar J. Lillevang's project files, compiled during the course of his career as a consulting engineer, and technical reports and specifications. Project files consist of correspondence, reports, drawings, blueprints, data, photographs, etc. Technical reports consist of documents written by Lillevang, and documents collected by Lillevang on a variety of topics. Project locations are primarily Southern California, but other parts of California and international locales are included as well.